The "Null" operation performs no processing. It merely propagates its
first source along the operation chain unmodified. There may be an
arbitrary number of sources but only the first one is passed along
so it must have the appropriate class type for the operation mode.

This operation may be useful as a placeholder in operation chains
and in creating nodes to which PropertyGenerators may be
attached. This would enable non-image data nodes to be present in chains
without requiring that specific OperationDescriptors be
implemented for these operations. The PropertyGenerators
required would in this case be added locally to the nodes using the
addPropertyGenerator() method of the node.

The region over which the data of two renderings of this
operation may be expected to be invalid or null
if there is no common region of validity.
A non-null empty region indicates that the
operation would produce identical data over the bounds of the
old rendering although perhaps not over the area occupied by
the tiles of the old rendering.